This bug was originally marked as duplicate of bug 191206, that is already resolved. This bug is still valid, so this seems to be another bug, but it might be related.
Build: NetBeans IDE 7.3 RC1 (Build 20130125-0fbf0f6115da)
VM: Java HotSpot(TM) Client VM, 23.6-b04, Java(TM) SE Runtime Environment, 1.7.0_11-b21
Created attachment 130836 [details]
This bug already has 5 duplicates
Created attachment 132814 [details]
changing insets of multiple selected object in the GridBag Customize layout dialog.
Created attachment 133768 [details]
Changed the insets of a component within an embedded GridBagLayout.
This bug already has 10 duplicates
Created attachment 133769 [details]
screenshot showing embedded GridBagLayout
Created attachment 133770 [details]
screenshot showing the component being adjusted
When fourth number in the insets was changed, the error dialog appeared.
Created attachment 133796 [details]
Removed the border from a JPanel in the GridBagLayout editor.
Created attachment 133797 [details]
Exception dialog appeared when closing the GridBagLayout editor.
Created attachment 137352 [details]
Created a JFRame
Set layout to GridbagLayout
DnD a number of JLabel and JtextFields
Opened the GridBag customizer
multi-selected the text field by using the mouse (dragging the mouse around the textfields)
Tried to move the selected field to a new cell
Looks like the fix of bug 191206 was later accidentally removed.